Cosmic Gamma-Ray Absorption Varies by Light’s Quirk
New Study Reveals Polarized Light Bends Cosmic Gamma-Ray Transparency
Astronomers have found that the absorption of high-energy gamma-rays from space is dependent on the light's polarization—the direction its electric field vibrates.
Scientists have long studied gamma-ray absorption, a process where a gamma-ray photon (a tiny packet of light) smashes into a low-energy photon, creating new particles. This absorption affects how we see distant gamma-ray sources like gamma-ray bursts (GRBs) and blazars.
Until now, scientists mostly considered this absorption without accounting for light's polarization. But what if the way light wiggles, its polarization, makes a difference?
Methodology and Findings
The researchers used a simple model of a gamma-ray source, similar to those in GRBs, but with a perfectly tidy magnetic field. They calculated how gamma-rays would be absorbed based on whether their electric fields vibrate parallel or perpendicular to this magnetic field.
- Using existing equations from 1934, they determined the gamma-ray absorption for these different light orientations.
Their investigation showed a subtle yet significant effect:
- Using the old "average" absorption numbers can overestimate the true absorption by up to 10 percent when the magnetic field is perfectly ordered.
- Specifically, gamma-rays vibrating parallel to the magnetic field get absorbed about 10 percent more than those vibrating perpendicular.
- This means the universe is slightly more transparent to some gamma-rays than previously thought.
- The study also predicts that the faint glow of polarization will increase by about 2.7 percent after some gamma-rays get absorbed away.
Implications and Future Research
Study author M. Böttcher clarified the practical implications:
"For realistic astrophysical scenarios with partially ordered magnetic fields, the use of the polarization-averaged γγ cross section is justified for practical purposes, such as estimates of minimum Doppler factors inferred for γ-ray bursts and blazars, based on γγ transparency arguments..."
This finding could help differentiate between shifts in gamma-ray light caused by absorption versus those from changes in the source itself.
Limitations
It's important to note the study's primary limitation:
The study used a simplified model with perfectly straight magnetic fields. Real cosmic magnetic fields are messy, which would likely lessen the polarization effect.
Future research will explore these more complex, realistic cosmic environments. This new understanding hints that studying the polarization of gamma-rays could offer a new window into the extreme environments of exploding stars and supermassive black holes.
Citation:
M. Böttcher, "The Polarization Dependence of γγ Absorption — Implications for Gamma-Ray Bursts and Blazars," arXiv:1409.1674v1 [astro-ph.HE] (2014), accepted for publication in The Astrophysical Journal.